HIGHLY INSULATING AND LIGHT TRANSMITTING
AEROGEL GLAZING FOR WINDOW
(HILIT AEROGEL WINDOW)
Karsten I. Jensen and Jørgen M. Schultz
DEPARTMENT OF CIVIL ENGINEERING (BYGDTU)
TECHNICAL UNIVERSITY OF DENMARK
Contract JOR3-CT97-0187
PUBLISHABLE REPORT
1 November 1998 to 31 October 2001
Research funded in part by
THE EUROPEAN COMMISSION
in the framework of the
Non Nuclear Energy Programme
JOULE III
HIGHLY INSULATING AND LIGHT TRANSMITTING
AEROGEL GLAZING FOR WINDOW
(HILIT AEROGEL WINDOW)
Edited by
Karsten I. Jensen and Jørgen M. Schultz
Department of civil engineering (BYGDTU)
Technical university of Denmark
2001
Contract JOR3-CT97-0187
Research funded in part by
THE EUROPEAN COMMISSION
in the framework of the
Non Nuclear Energy Programme
JOULE III
ABSTRACT
The HILIT AEROGEL WINDOW project with participants from Denmark (coordinator),
France, Germany, Norway and Sweden, was formulated in order to develop a safe and clean
production of monolithic silica aerogel based on supercritical CO2 drying of the gels, to study
the process parameters and to transfer the results from lab- to mid- and finally to large-scale
making of 60 by 60 cm2 in a pre-industrial plant. The large samples forms the basis for
assembly of evacuated aerogel glazings optimised with respect to thermal and optical
properties.
The production process development and transfer to pre-industrial fabrication of aerogels has
succeeded in all details. A pilot plant for precursor elaboration has been established and
precursors of required amount and quality has been delivered to all partners. Studies at labscale have identified the important parameters for optimising the mixing of the chemicals,
which is the mixing rate and the HF (catalyst) flow rate. A mixing reactor have been designed
and successfully transferred to large-scale application. A wet gel strengthening process has
been developed and optimised at laboratory-scale and transferred at mid-scale with success
(concerning monolithicity). A direct supercritical CO2 drying loop has been developed at midscale, successfully transferred and re-adapted at large-scale. At large scale a complete CO2
loop has been build including CO2 regaining by separation of CO2 from the solvent. The CO2
has been reused for several batch runs. The up scaling required invention of several technical
solutions related to moulding and handling of the large gels. Despite the efforts only aerogels
with a thickness up to 15 mm have been produced with a good reproducibility. The thermal
conductivity is approximately 0.015 W/mK at atmospheric pressure and 0.010 W/mK at 10
hPa. The optical properties have been improved compared to previous aerogels thanks to the
process and the smooth surfaces obtained and a heat treatment of the dried aerogel.
A rim seal solution that offers the required air and moisture tightness without leading to
severe thermal bridge effects has been developed as well as an assembly process including
heat treatment and evacuation in a vacuum chamber. The centre U-value is measured for
several prototype glazings to 0.68 W/m2K, which is somewhat higher than the targeted value
of 0.4 W/m2K due to the thinner aerogel sheets available. The overall U-value including the
thermal bridge effect of the rim seal solution is measured to 0.74 W/m2K. The solar energy
transmittance is measured to 76% thanks to the use of low-iron glass with an anti reflective
coating.
TABLE OF CONTENTS
1.
PARTNERSHIP .......................................................................................................... 2
2.
OBJECTIVES OF THE PROJECT .......................................................................... 3
3.
3.1
3.2
3.2.1
3.2.2
3.2.3
3.2.4
3.2.5
3.3
3.4
TECHNICAL DESCRIPTION ..................................................................................
Introduction .................................................................................................................
The process operation .................................................................................................
Precursor synthesis ........................................................................................................
Solvent mixing studies ..................................................................................................
Washing and ageing ......................................................................................................
Drying studies ................................................................................................................
Transfer to large scale ...................................................................................................
Characterisation .........................................................................................................
The glazing ...................................................................................................................
4.
4.1
4.1.1
4.1.2
4.1.3
4.1.4
4.1.5
4.1.6
4.1.7
4.2
4.3
4.3.1
4.3.2
4.3.3
4.3.4
4.3.5
4.3.6
RESULTS AND CONCLUSIONS ............................................................................. 7
The material ................................................................................................................. 7
Precursor (PCAS) .......................................................................................................... 8
Strengthening washing and ageing (LACE + NTNU) .................................................. 8
Direct supercritical CO2 drying process (ARMINES) .................................................. 9
Large scale aerogels (AIRGLASS) ............................................................................. 10
CO2/ETAC separation and CO2 regaining loops (AL GAS) ....................................... 11
Post processing (AIRGLASS) ..................................................................................... 12
Future aspects to be improved ..................................................................................... 12
Characterisation of the optimum aerogel window (CSTB + FHG.ISE) ............... 13
Glazing ........................................................................................................................ 14
Rim seal solution (BYGDTU) ................................................................................... 14
Evacuation and assembly process (BYGDTU) ......................................................... 16
Glazing optimisation (BYGDTU) ............................................................................. 16
Measured performance (BYGDTU) .......................................................................... 16
Energetic interest analysis (ARMINES) ..................................................................... 17
Visual comfort analysis (FHG.ISE) ............................................................................ 18
5.
EXPLOITATION PLANS AND ANTICIPATED BENEFITS ............................ 19
6.
REFERENCES .......................................................................................................... 20
4
4
5
5
5
5
5
6
6
6
APPLICATION RELATED FIGURE ................................................................................ 21
1
1.
PARTNERSHIP
The following organisations have participated in the project:
Contractors
Department of Civil Engineering
Technical University of Denmark
Building 118, Brovej
DK-2800 Kgs. Lyngby
DENMARK
Centre d’Energétique
Ecole des Mines de Paris
Sophia Antipolis. B.P. 207
F-06560 Valbonne
FRANCE
Produits Chimiques Auxiliaires et de
Synthéses (PCAS)
B.P. 111
F-91161 Longjumeau
FRANCE
CSTB
24, rue Joseph Fourier
F-38400 Saint Martin d’Hères
FRANCE
Laboratoire d’Application de la
Chimie à l’Enviromnement (LACE)
Université Claude Bernard Lyon 1
(UCBL)
43, boulevard du 11 Novembre 1918
F-69622 Villeurbanne Cedex
FRANCE
Fraunhofer Institute for Solar Energy
Systems (ISE)
Heidenhofstr. 2
D-79110 Freiburg
GERMANY
Airglass AB
Box 150
S-24522 Staffanstorp
SWEDEN
Air Liquide Gas AB
Lundavegen 147
S-21224 Malmø
SWEDEN
Abbreviation Contact person
Dr. K.I. Jensen
Tel: (45) 45 25 18 90
BYGDTU
Fax: (45) 45 88 32 82
E-mail: kij@byg.dtu.dk
ARMINES
Dr. P. Achard
Tel: (33) 4 93 95 75 08
Fax: (33) 4 93 95 75 35
E-mail: patrick.achard@cenerg.cma.fr
PCAS
Dr. M. Durant
Tel: (33) 1 69 09 77 85
Fax: (33) 1 64 48 23 19
E-mail: Marcel.Durant@pcas.fr
CSTB
Mr. B. Chevalier
Tel: (33) 4 76 76 25 56
Fax: (33) 4 76 76 25 60
E-mail: br.chevalier@cstb.fr
LACE
Prof. G.M. Pajonk
Tel: (33) 4 72 44 82 52
Fax: (33) 4 78 94 19 95
E-mail: pajonk@univ-lyon1.fr
FHG-ISE
Dr. P. Nitz
Tel: (49) 761 45 88 5410
Fax: (49) 761 45 88 9410
E-mail: nitz@ise.fhg.de
Dr. L. Gullberg
Tel: (46) 46 25 52 00
AIRGLASS
Fax: (46) 46 25 69 20
E-mail: info@airglass.se
Mr. M. Rydén
Tel: (46) 40 38 11 17
AL GAS
Fax: (46) 40 93 19 77
E-mail: Mats.Ryden@AirLiquide.com
2
The project has been organised as shown below:
Project Organisation and Task responsibilities
HILIT AEROGEL WINDOW
Project Coordination
BYGDTU
TASK 1
Process operation and
evaluation
ARMINES
AL GAS
LACE
PCAS
AIRGLASS
2.
<=>
TASK 2
Material characterisation
CSTB
FHG-ISE
<=>
TASK 3
The glazing
<=>
BYGDTU
CSTB
FHG-ISE
ARMINES
OBJECTIVES OF THE PROJECT
The main project objectives are: 1) to make the pre-industrial elaboration process evaluation
for the chemical part (aerogel process) and the glazing assembly process of aerogel window
and 2) to estimate the energetic interest of such glazings.
Other objectives are:

Flat aerogel sheets of about 60 x 60 x 2 cm3 with a solar transmittance of 90 % or more
and the lowest heat conductivity ever reported for such material.

The aerogel material exhibits only negligible image blur ie the optical quality is at the
same level as ordinary glass panes.

Prototypes of 60 x 60 cm² evacuated aerogel glazing (aerogel thickness of 20 mm) made
by the proposed process and having a centre heat loss coefficient (U-value) below 0.4
W/m² K, an overall U-value below 0.5 W/m² K and a solar energy transmittance (gvalue) above 75 %. Finally, the lifetime of the glazing with respect to maintaining the
gas pressure below 50- 100 hPa will be at least 30 years.

Feasibility study, which analyses the technical potential of aerogel windows.

Technical and economic evaluation of an industrial production of aerogel glazings for
windows.

Analysis of market potential of aerogel windows taking an integrated design approach
into account.
3
At the end of the project, it is expected that the following are developed: A supercritically
CO2 based process with recovery, that can give flat aerogel tiles of 60 cm square as well as a
glazing assembly process, suited for an industrial production, for the same size. And finally, a
general scheme for demonstration of the new technology will be drafted.
3.
TECHNICAL DESCRIPTION
3.1
Introduction
Aerogels were first made by Kistler in the early thirties [Kistler, 1932]. To avoid dissolution
of silica during the supercritical drying he exchanged the pore liquid (mainly water) with
ethanol. This time consuming exchange of the pore liquid was later avoided by developing a
procedure where alcogels directly prepared from alcoxides were supercritical dried from
ethanol or methanol. [Teichner, 1972; Henning, 1986]. Due to the high supercritical
temperature of alcohols, a new and safer route consisting of an exchange of the pore liquid
with CO2 followed by a drying at the supercritical conditions of CO2 was developed [Tewari,
1986]. Later, direct supercritical CO2 washing was tested to improve the diffusion in the
nanoporosity of the wet gel [Van Bommel, 1995]. Since then, few studies had been performed
in the aerogel field with such an innovative process [Knez, 1998]. The obtained results are
very promising, however, these processes have still not shown to be preferable for a
production of large aerogel sheets. This is the reason why the direct supercritical CO2 drying
combined with a patented gel preparation [Pajonk, 1998] and a wet gel-strengthening step
[Einarsrud, 1994] was chosen for the work in this project.
Aerogels not only offers a very low thermal conductivity but also a very high light and solar
energy transmittance, that makes aerogel glazing superior to other super insulating glazing
types on the market.
The key points related to development of super insulating aerogel glazing are primarily to be
able to evacuate the aerogel to approximately 1 hPa and to maintain the internal pressure in
the glazing below 50 hPa within the targeted lifetime of more than 25 years.
Aerogel has at atmospheric pressure a thermal conductivity of 15 – 20 mW/(m K) but
evacuation to a pressure below 50 hPa eliminates the conductive heat transfer in the pores and
the thermal conductivity becomes as low as 8 mW/(m K). Whit this low conductivity a centre
U-value of 0.4 W/(m2 K) could be reached with an aerogel thickness of 20 mm along with a
solar energy transmittance of approximately 75%.
The aerogel should be encapsulated in an airtight shell in order to maintain the low internal
pressure. On the outside and inside surface of the aerogel, glass panes can be used as a
completely airtight cover, leaving the rim seal as the crucial part. The rim seal solution should
both have the sufficient air tightness and a high thermal resistance that matches the very good
performance of the evacuated aerogel. Finally, the rim seal solution need to be flexible as the
aerogel is slightly compressed when evacuated due to the external atmospheric load.
Tempered glass is expensive compared to non-tempered glass so a special point of interest is
if tempered glass can be avoided. This requires completely flat aerogel sheets and the
4
development of new rim seal solutions that ends up with the same thickness as the evacuated
aerogel.
The goal is to optimise the different steps in the assembling process always focusing on
achieving a fast, economic and reliable process.
3.2
The process operation
3.2.1
Precursor synthesis
General synthesis of PCAS precursor is a sub-stoechiometric hydrolysis of tetraethoxysilane
(TEOS) in ethanol under H2SO4 catalysis. The slow polycondensation mechanism in acidic
conditions carries on for few weeks after synthesis. As the reaction proceeds, the polysilicates
grow in molecular weight and chain length, until most of or all of the ethyl groups are driven
off and a non-linear network of Si-O-Si remains. This chemical process of hydrolysis is the
basis of Ethyl silicate products. Under careful conditions, a stable mixture of polysilicate
“pre-polymers” can be obtained.
Within this project the work on the precursor has been dedicated to be able to make the
required amount of precursor for large-scale production and make sure that the precursor can
withstand transportation and storage.
3.2.2
Solvent mixing studies
To facilitate wet gel synthesis at large-scale (i.e. at AIRGLASS place), a specific sub-task has
been created at laboratory scale (LACE). It aims at studying the evolution of gelation time
with some of the main mixing parameters.
3.2.3
Washing and ageing
During the direct supercritical drying process using CO2, the mechanical properties of both
the wet and dried gels are of importance. Even if the direct supercritical CO2 drying reduces
capillary stresses [Reid, 1987], it is interesting at large scale to increase both the modulus of
rupture (MOR) and permeability (D) of the wet gel in order to diminish the impact of
depressurisation stresses [Woignier, 1994]. The objective of this sub-task was to investigate
the strengthening of wet gels to ensure success in obtaining large monolithic volumes through
an acceptably fast process by including a washing and an aging step prior to the supercritical
drying.
3.2.4
Drying studies
A direct supercritical loop was implemented and operated at mid-scale (ARMINES) first to
demonstrate the experimental feasibility of the direct supercritical drying process, then to
facilitate scaling-up at large-scale and also to validate studies on precursor and lab-scale
improvements. To improve the efficiency of the supercritical washing phase, the
5
measurement of the ETAC concentration in the autoclave, during this step, is of the utmost
importance both for material (aerogel monolithicity,…) and process (reduction of the duration
of the process, …) aspects.
On-line measurements of the composition of the high-pressure media present in the autoclave
are performed by Gaseous Phase Chromatography (CPG). They concern ETAC and CO2. The
mixtures are injected in the CPG thanks to a Rapid On Line Small Injector apparatus
(ROLSITM, technology developed by ARMINES [Armines, 1986], [Baba-Ahmed, 1998.
Associated with prior investigated CO2 and ETAC sampling curves and analytical
calculations these “on line” CPG measurements permit to estimate the evolution of the ETAC
concentration in the autoclave during the drying process (CETAC(t)).
3.2.5
Transfer to large scale
The main objective at large-scale consists in the development at AIRGLASS AB of a clean
and safe production process of large, super-flat, monolithic aerogel sheets with a constant
thickness, a smooth surface and good thermal and optical properties. The process comports
four main steps concerning.




wet gels elaboration,
direct supercritical CO2 drying,
CO2 regaining,
solvent handling and recycling.
The scaling-up process involves a lot of technical problems to overcome, which is not
encountered at lab and mid-scale level. It mainly concerns safety problems and handling of
the large gels.
3.3
Characterisation
The success of the project relies on a close link between the material process experiments and
the resulting material parameters in order to know which way to continue and optimise the
process. So, a special task was dedicated to characterisation of the aerogel samples produced
mainly with respect to optical and thermal parameters.
Beside the characterisation it was attempted to identify the links between the aerogel structure
and the thermal and optical properties in which way simulation models could be used for
further improvements of the aerogel.
3.4
The glazing
Achievement of the excellent thermal parameters of aerogel glazings requires a rough
evacuation of the aerogel to a pressure below 50 hPa. In order to keep the vacuum the aerogel
need to be enclosed in an airtight envelope. On the inner and outer surface glass panes offers
the required airtightness leaving the edges as the remaining problem. A metal or glass profile
could make the glazing completely airtight but the thermal bridge effect would completely
destroy the good insulating properties of the aerogel. The main goal is to develop an airtight
rim seal solution that only leads to an insignificant additional heat loss.
6
The rim seal solution should be designed so the final glazing becomes completely flat after
evacuation. In this way no critical stress in the glass panes will occur and ordinary nontempered glass can be used, which considerably will lower the glazing costs.
Finally the glazing should be optimised with respect to light and solar transmittance through
investigation of available glazing types and thickness as well as enhancement of the
transmittance by means of surface treatment of the glass panes. The targeted value for the
total solar energy transmittance is 75%.
The assembly will take place in a vacuum chamber made as part of a national Danish project
[Jensen, 1999]. Optimisation should be performed concerning the required evacuation time in
order to cut down the total assembly time. The most important parameter is foreseen to be the
effective diffusion coefficient of the aerogel.
However, the overall goal for the application task is to produce evacuated glazings based on
the optimised rim seal and the optical improved glass covers and to prove the concept and
reproducibility of the assembling process.
The produced glazings is characterised with respect to overall U-value and solar energy
transmittance and will serve as “promotion” samples towards architects and glazing
manufacturers. The final optimised glazing forms the basis for an outline for demonstrating
the possibilities with the present aerogel glazing quality, which seems as the only way to get
the industry and in particular the architects interested.
The measured thermal and optical parameters will form the input for a detailed building
simulation program in order to quantify the energetic interest for aerogel windows in different
climates (by comparisons with reference glazings) and to evaluate the influence on daylight
and thermal comfort.
4.
RESULTS AND CONCLUSIONS
4.1
The material
The permanent collaboration between partners, and more particularly within the Task 1 frame,
has permitted to converge in the vicinity of the general Task 1 objectives. Indeed, the two
following points can be stressed.
 Large monolithic and flat silica aerogels, presenting thermal and optical properties close to
those permitting to reach the expected aerogel glazing properties, have been elaborated.
 A clean and safe elaboration process including moulding, direct supercritical CO2 drying,
CO2 recovery and ETAC handling has been developed at large-scale.
Whatever the elaboration scale, the typical results are the following ones. The produced
aerogels are light (0.150 g/cm3  0.1) and mesoporous materials. Their thermal conductivity
at room temperature and atmospheric pressure of air is close to 0.016 W/m.K (while inferior
7
to 0.010 W/m.K at 10-1 mbar). 15 mm thick-aerogels present a normal-hemispherical percent
of transmission and an extinction coefficient respectively close to 85 % and 15 m-1.
From the process point of view, the following main results can be underlined. The main
contractor is mentioned in parentheses after each result.
4.1.1
Precursor (PCAS)
P750 precursor has been elaborated at a pilot scale (100 litres-batch) without significant
reproducibility hardships.
4.1.2
Strengthening washing and ageing (LACE + NTNU)
A specific strengthening washing and aging process has been developed close to room
temperature, leading to a simultaneous increase of the permeability and the modulus of
rupture. It has been successfully enlarged from small to mid scale (i.e. from cylinder with
diameter and thickness respectively equal to 3.5 and 1 cm to 1061 cm3).
Modulus of rupture, MOR (MPa) Permeability, D (nm2)
18
16
14
12
10
Washing
Aging
8
6
0.229
0.25
Washing
Aging
0.224
0.2
0.178
0.15
0.222
0.173
0.229
0.175
0.176
0.1
0.05
0.175
0
5
10
15
20
25
Washing time in 20 vol% H2O/EtOH at 60°C (h)
Figure 2 Modulus of rupture (MOR), and permeability (D) of wet gels as a function of
washing and aging time. Data for washed only gels are given as open symbols and
data after aging as closed symbols. The numbers on the data points indicates the
initial density of the gels. The shear modulus followed a similar behaviour as the
MOR.
8
4.1.3
Direct supercritical CO2 drying process (ARMINES)
A direct supercritical CO2 drying process process (including mass-flow rate and
depressurisation control) has been developed at mid-scale, leading to perfectly monolithic
13131 cm3 aerogels (figure 3) with a high reproducibility ratio. It has been transmitted to
large-scale (i.e. from 13131 to 55551.5 cm3).
Figure 3
Aerogel obtained at ARMINES on NTNU wet gel (Washing in 20 vol%
H2O/EtOH, 8 h 60°C; Aging in 20 vol% P750/EtOH, 7 h RT)
A method based on the ROLSITM sampling apparatus and Gaseous Phase Chromatography
has been implemented and tested at mid-scale to study the advancement of the supercritical
washing phase in order to optimise the “end-of –drying” criteria (figure 4). It has been studied
to be transferable for the control of large-scale drying process.
In parallel, static Vapor Liquid Equilibrium curves of the CO2/ETAC system have also been
measured at a supercritical (35°C) and two sub-critical temperatures (10 and 20°C) to
facilitate drying and regaining studies at large-scale.
ETAC pea
k
t =t112 min (0.2 h)
t t2
= 108 min (1.8 h)
Residual
peak
t =t31133 min (18.8 h)
t =t42757 min (46 h)
Figure 4
Evolution of the organic peaks during supercritical washing.
9
4.1.4
Large scale aerogels (AIRGLASS)
A specific moulding process between two glass-sheets has been developed at large-scale,
leading to ultra-flat, monolithic wet gels with an excellent surface structure and a high
reproducibility ratio.
When mixing the chemicals at large-scale, consideration has to be taken that hydrofluoric acid
(HF) is one of the components. The safety of the personal has to be considered and the mixing
equipment has to be resistant against HF. It is reminded that the recipe is 50 vol% P750, 48
vol% ETAC and 2% HF (21N). Because of large-scale requirements, HF is premixed with
equal volume of EtOH (to increase solubility in ETAC). The chemicals are held at room
temperature. The ETAC is poured into the mixing reactor, and the HF-EtOH mixture is added
during vigorous stirring. After 5 min stirring the precursor is poured into the HF-EtOH-ETAC
mixture. The mixing time is tried out and calculated for each precursor batch and HF quality.
When the mixing is done, the sol is poured into the mould. At the same time the mould is
lowered into a tank with tempered water (25°C). This is performed to equalize the pressure
inside and outside the mould. Control of the gelation time is done. After gelation, syneresis
phenomena (hydrolysis and condensation) occur during one whole day before any new
manipulations of the wet gel are done.
Moulding scheme at AIRGLASS
Deforming scheme at AIRGLASS
Storing scheme at AIRGLASS
Figure 5 Moulding and storing at AIRGLASS.
The wet gel-mould is placed in an ETAC bath. The ETAC level has to completely cover the
mould to avoid evaporation and the appearance of cracks. With help of special but simple
10
equipment the upper glass sheet is carefully removed. After a few minutes the wet gel sheet
will float, depending on that its density is slightly lower than the density of ETAC, because of
presence of water and ethanol coming from the syneresis phenomena. After a while, some of
the water and ethanol is washed out and replaced with ETAC by molecular diffusion. Then
the density of the wet gels sheet increases and the sheet sinks onto the drying form.
An alternative is to not remove the glass sheets until the wet gels has completely hydrolysed
for about few days, depending on the concentration of HF vol% in the sol.
The drying form with the wet gels is removed from the ETAC bath after typically two to three
days and placed in a cassette, which is used in the diffusion reactor during drying. The
cassette is stored in a container with tempered ETAC (20°C). The purpose of storing the wet
gels in ETAC is to redraw as much water and ethanol as possible and replace it with ETAC
before the sheets are processed in the autoclave The container is used for a safe transporting
of the sheets from the moulding station to the autoclave. Because of continuation of syneresis
reactions, the strength of the wet gels is also observed to increase. Handling the gels as
described above has proven to result in 60601.5 cm3 wet gels without monoliticithy
damage. Direct supercritical CO2 drying has been operated at large-scale with success.
Whenever all the other required characteristics are obtained (wet gel monolithicity and purity,
CO2 purity, suited drying forms, etc.) it leads to large monolithic aerogels.
4.1.5
Figure 6
CO2/ETAC separation and CO2 regaining loops (AL GAS)
Sketch of the total plant at AIRGLASS.
11
4.1.6
Heat treatment (AIRGLASS)
A heat treatment of the aerogels has been successfully achieved, leading to an improvement of
the optical quality of the aerogel (figure 7).
Figure 7 Evolution of the aerogel colour with temperature level of the heat-treatment
4.1.7
Future aspects to be improved
Despite the large improvements obtained many different aspects still remain to be improved.
They mainly concerns:







Scaling-up of the strengthening process from mid to large-scale.
Improvements of the monolithicity for aerogels of 20 mm thickness.
Industrial adaptation of the production of wet gels and reduction of the total time for
mixing, moulding, gelation and storing.
Reduction of the duration of the whole drying process.
Improvement of the (ETAC-Ethanol-Water)/CO2 separation process.
ETAC recycling at large scale.
Elimination of marks at the surface of the aerogel by performing vertical drying of the
samples.
The present studies have permitted to elaborate strategies for further improvements of each of
these points.
In particular, concerning time reduction, the following works must be investigated.



Optimisation and concluding of the depressurisation and “end-of-drying” criteria studies
and adaptation to large-scale.
Studies and development concerning turbulent CO2 flux for drying of the wet gel.
Continuous dynamic supercritical CO2 drying of the wet gels at large scale, followed by a
systematic heat-treatment step of the aerogels.
12
4.2
Characterisation of the optimum aerogel window (CSTB and ISE.FHG)
Characterisation of samples and prototypes has been performed throughout the project. Here
is only shown a few examples of the measurement results.
Figure 8 shows the measured visual and solar transmittance as function of angle of incidence
as measured by FHG.ISE.
Comparison of Aerogel glazing prototypes
0.8
0.7
transmittance
0.6
0.5
0.4
VIS Prototype No.8
0.3
SOL Prototype No.8
VIS unevac. glazing sheet 27
0.2
SOL unevac. glazing sheet 27
0.1
0.0
0
10
20
30
40
50
60
70
80
90
angle of incidence / deg
Figure 8 Angular dependence of solar and visual transmittance for two aerogel glazing
prototypes.
Beside the optical and thermal characterisation CSTB also measured the sound reduction
capabilities of aerogel glazings. Figure 9 shows the test glazing and table 1 shows the results
compared to ordinary glazing types.
Figure 9 Aerogel test glazing for acoustic characterisation. The valve in the centre is used to
control the internal pressure.
Table 1
Fading index R for aerogel glazing and 2 different commercial double-glazing. All
glazings measures 0.5  0.5 m2.
Glazing type
Pink noise reduction
Traffic noise reduction
4 mm glass, 12 mm air, 4 mm glass
48
43
4 mm glass, 6 mm air, 10 mm glass
51
46
13
4 mm glass, 12 mm aerogel, 4 mm glass
40
39
The sound reduction is not as good as sealed glazing units with a gas filled enclosure and it
may be necessary to add a third layer of glass in combination with the aerogel glazing with an
air layer in between.
4.3
The glazing
The total process of making an evacuated aerogel glazing is shown in figure 11. The main
results are described below.
4.3.1
Rim seal solution (BYGDTU)
It has succeeded to develop a rim seal solution with a minimal thermal bridge effect based on
a laminated 70 m thick foil developed by Dupont for vacuum insulation purposes. The foil is
sealed against the glass covers with a very thin butyl layer compressed between the glass and
a polystyrene spacer during the assembly and afterwards due to the external atmospheric
pressure on the glass panes. The foil is applied in a protected position between the aerogel and
Foil
Foil
1
Polystyrene
Butyl
2
3
3
Polystyrene
1
2
Aerogel
Foil
Foil
Polystyrene
Butyl
the polystyrene spacer.
Figure 10 Sketch of the rim seal assembling.
The polystyrene spacer offers the required compression strength for the compression of the
butyl sealant and by a careful choice of spacer height any bending of the glass covers has been
avoided and non-tempered glass panes have been used for all prototypes without any
difficulties.
14
The described solution is calculated to have a lifetime with respect to gas and moisture
diffusion of more than 30 years.
15
1) Heat treatment at 425 °C.
2) Rim seal of foil and polystyrene spacer.
3) Lower glass + aerogel + rim seal.
4) Upper glass fixed to lid of vacuum
chamber by means of electromagnets.
Piston
Weight
5) Lower glass + aerogel + rim seal in
vacuum chamber.
6) Vacuum chamber closed and upper
glass pressed against rim seal and
aerogel by means of the piston.
7) Aerogel glazing in the chamber after
evacuation.
8) Final aerogel glazing.
Figure 11 Illustration of the different steps in the assembling process.
16
4.3.2
Evacuation and assembly process (BYGDTU)
75% of the evacuation time is due to the very small diffusion coefficient of the aerogel even
that the evacuation takes place through the large surface of the aerogel. The diffusion
coefficient have been measured to approximately 2.5  10-6 m2/s. Evacuation of a 15 mm
thick aerogel sample takes about 30 minutes to reach a pressure of 5 hPa in the aerogel and is
independent of the vacuum pump capacity.
Six optimised aerogel glazings have been produced with the described rim seal and assembly
procedure – all showing a good airtightness.
4.3.3
Glazing optimisation (BYGDTU)
Beside the rim seal development, which also is a thermal optimisation of the glazing, use of
low-iron glass and an anti reflective coating of the glass panes has resulted in aerogel glazings
with a total solar energy transmittance above 75% for a 17 mm aerogel glazing. Furthermore,
the heat treatment process developed by AIRGLASS has been implemented and scaled up at
(BYGDTU) and shortened considerably thanks to a powerful oven. The total heat treatment
only takes 60 minutes.
The heat treatment also removes all water from the aerogel sample, which is crucial with
respect to avoidance of internal condensation when the glazing is exposed to severe
temperature gradients e.g. in the northern Scandinavia with –40 °C outside.
4.3.4
Measured performance (BYGDTU)
The centre U-values of the optimised glazing prototypes have been measured by means of a
hot plate apparatus. The average centre U-value is found to be 0.66 W/m2K, which with the
average aerogel thickness of 14.8 mm correspond to an average thermal conductivity of 0.010
W/mK. This indirect determined thermal conductivity is in accordance with the measured
material properties at a pressure level of 1-10 hPa.
Figure 12
Four optimised aerogel prototypes joined in a test frame for hotbox
measurements.
17
Four of the optimised prototypes have been used for a test window (figure 12) measuring 1.20
by 1.21 m2 designed for hotbox measurements of the overall U-value. The well-insulated
framing system is made only for fixation of the four glazings and will not withstand exposure
to real climate for longer periods. The measured overall U-value of the glazing is deduced
from the measurements by subtracting the heat loss through the framing system. The result of
an average total U-value of 0.74 W/m2K compared to the average centre value of 0.66 W/m2K
confirms the very small thermal bridge effect of the developed rim seal solution.
4.3.5
Energetic interest analysis (ARMINES)
Simulations of the energetic performance based on the measured U-values and the measured
optical properties have been carried out for different climates. The general conclusion is that
aerogel windows are most suited for northern climates with low temperatures and little
sunshine during the heating season. In southern climates overheating and thus cooling needs
diminish the advantage of the good insulating properties or even leads to a little higher energy
consumption.
North facing aerogel glazing shows excellent possibilities for achieving more daylight
without increased energy consumption for heating
Stockholm / Cooling/ South 40%
Stockholm / Cooling/ South 20%
Annual Energy needs - Stockholm - 40%
Annual energy needs - Stockholm - 20%
3000
KWh 2500
2500
2000
KWh
2000
Reference40
1500
Reference20
1000
PRT12
Aerogel20
PRT12
1000
500
500
0
Wheating
Wcooling
Wlighting
0
Wtotal
Wheating
Stockholm /No cooling/ North 40%
Wcooling
Wlighting
Wtotal
Stockholm / No cooling/ North 20%
Annual energy needs - office building facing North -
Annual energy needs - office building facing North -
40% glazing aera- Stockholm -
20% glazing aera- Stockholm -
2500
3000
2500
2000
2000
Reference40
1500
KWh
KWh
1500
Aerogel40
Aerogel40
1000
PRT12
Reference20
1500
Aerogel20
PRT12
1000
500
500
0
0
Wheating
Wlighting
Wtotal
Wheating
Wlighting
Wtotal
Figure 13 Comparison of annual energy needs with reference glazing and new aerogel glazing
(Sample PRT12 elaborated with RUN9-ID56)
The slight diffusing of the daylight passing through an aerogel glazing result in a better
daylight distribution and quality with reduced glare.
18
4.3.6
Visual comfort analysis (FHG.ISE)
The measured optical properties have been used for simulations with the program
RADIANCE of the visual comfort with aerogel windows compared to common heat mirror
glazed windows. The results are shown in figure 14. They clearly show the diffusion of direct
light on the aerogel glazing.
Figure 14
a)
b)
c)
d)
a) office room with a south facing facade equipped with double heat mirror
glazing. On a clear day, direct sunlight is incident at an angle of 45° directly
from the south.
b) same situation as in a) but facade equipped with aerogel glazing. Bright
sunlight on the glazing is a "worst case" situation concerning both color shifts
and contrast reduction of the view outside.
c) same office room as in a) for a day with a bright overcast sky. No direct
sunlight is incident on the facade.
d) same situation as in c) but facade equipped with aerogel glazing. The contrast
reduction and colour effects are significantly weaker than in b) with direct
sunlight on the facade.
19
5.
EXPLOITATION PLANS AND ANTICIPATED BENEFITS
The main focus within this project has been on achieving monolithic silica aerogel samples
produced at large-scale in a safe and clean way with good insulating and optical properties.
There is a strong interest from the community to achieve better insulating windows to reduce
the energy consumption for space heating and reduce the emission of green house gasses.
The performed work has lead to substantial improvement of the optical quality of the aerogel
material removing the former image distortion when looking trough an aerogel glazing, but
still aerogels will look hazy if exposed to direct light. This is seen as one of the most serious
restrictions for a widespread dissemination of aerogel glazings, as it will not be accepted as a
substitution of ordinary glazings.
On the other hand the diffusion of the daylight offers a more pleasant daylight distribution
with less glare, and the special appearance of the aerogel glazing could offer the architects
new possibilities in creating exciting buildings. And not to forget, that the improved daylight
utilisation can be achieved without raising the energy bill for heating. In fact even in a Danish
climate, north-facing windows with aerogel glazing will contribute to the heating of the
building seen over the heating season. From this point of view there should be an important
market in daylighting components that could replace opaque building components lowering
the use of artificial lighting and increase the quality of the indoor environment.
The existing industrial plant at AIRGLASS is only capable of making aerogels of
approximately 60 by 60 cm2, which also will be the final size of the individual glazing. More
aerogel tiles could be joined into one glazing, but the joints between the aerogel samples will
always be visible and need to be covered by some sort of framing system. This limited size of
aerogel glazings is also a serious limitation that perhaps even plays a major role for the
dissemination compared to the optical properties. Most of all because the large required
framing area increase the costs dramatically.
The making of aerogel glazing has now reached a level, where an up scaling of the production
plant and decrease of the production time is the main problems to overcome for advancing to
a real industrial production, which is required if the costs of silica aerogel should become
compatible.
In general the glazing manufacturers are not sure of the market potential and thus do not wish
to invest in new large production facilities. If this should be changed it is necessary to create a
demand. The most probable way for creating such a demand is to get the architects interested
in the new possibilities that aerogel glazing offers with respect to enhanced use of daylight
without bothering of increased energy consumption, down draughts and risk of condensation,
which often is seen e.g. in combination with skylights etc.
Based on the above discussion exploitation of the results from this project is foreseen to take
place in a demonstration project suited for the limited capacity of the pre-industrial
production plant at AIRGLASS, which also has been the intention when formulating the
project. An industrial designer and architect has been working on outlining the most
promising options for demonstration based on the present optical quality and size of the
aerogel glazings. This work will be used as basis for trying to realise a demonstration project
through contacts to possible customers.
20
6.
REFERENCES
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S.S. Kistler, J. Phys. Chem. 63 (1932), 52
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[Reid, 1987]
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[Baba-Ahmed, 1998]
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K. I. Jensen. Evacuation and assembly of aerogel glazing (in Danish).
Department of Buildings and Energy, Technical University of
Denmark. Technical report SR-9923. 1999
21
APPLICATION RELATED FIGURE
22